1. Field of the Invention
The present invention relates to transmission devices, transmission methods, and computer-readable media. In particular, the present invention relates to a transmission device, a transmission method, and a computer-readable medium capable of appropriate control of a transmission rate.
2. Description of the Related Art
In recent years, wireless networks, such as wireless local area networks (LANs) conforming to Institute of Electrical and Electronic Engineers (IEEE) 802.11, come into wide use instead of wired networks because the wireless networks have the advantages of, for example, higher flexibility in sites where devices are installed. As the result of various applications that process a large amount of data, such as images, and that frequently access the networks, higher transmission rates are required for the wireless LANS and it becomes necessary to further improve the throughputs.
In order to achieve the above object, IEEE 802.11 adopts transmission rate control in which multiple kinds of transmission rates are made available in the physical layer (PHY layer) and an optimal transmission rate is selected in accordance with a variation in the actual communication environment. Such transmission rate control is called link adaptation. For example, when the actual communication environment in a network is varied in a manner shown in FIG. 1 (refer to a dotted line in FIG. 1), the transmission rates used in the network are reset so as to agree with the actual communication environment (refer to solid lines in FIG. 1).
As a result, a higher transmission rate is selected to realize a higher throughput when the reception state is good whereas a lower transmission rate (that is, a transmission rate having a higher error resilience) is selected to improve the accessibility of transmission data and to improve the throughput (increase the amount of transmitted information) per unit time when the reception state is not good. The transmission rate is realized by a combination of the modulation method corresponding to the transmission rate, the redundancy, and so on. In Multiple Input Multiple Output (MIMO) communication systems conforming to, for example, Enhanced Wireless Consortium (EWC) or Worldwide Interoperability for Microwave Access (WiMAX), the numbers of antennas are also considered.
In a typical link adaptation in related art, a terminal at the reception side (hereinafter referred to as a responder) feeds back information (hereinafter referred to as feedback information) indicating the reception state of a signal received from a terminal at the transmission side (hereinafter referred to as an initiator) to the initiator, and the initiator resets the transmission rate on the basis of the feedback information to transmit next data. Such a method is called rate feedback. This method utilizes the characteristic in which the communication environment at the reception time at the responder is similar to that at the transmission time at the initiator when the reception time is close to the transmission time.
For example, in a system disclosed in Japanese Unexamined Patent Application Publication No. 2000-151639, a responder measures the communication environment of the own device on the basis of, for example, a received signal strength indicator (RSSI), upon reception of a Request to Send (RTS) packet transmitted from an initiator, adds the transmission rate corresponding to the measurement result to a Clear to Send (CTS) packet as feedback information, and returns the transmission rate added to the CTS packet to the initiator. The initiator sets the transmission rate on the basis of the feedback information described in the CTS packet returned from the responder.
In the network system, as shown in FIG. 2, a transmission inhibited period (the period of a Network Allocation Vector (NAV)) is set on the basis of information described in the Duration field of the RTS packet or the CTS packet. The transmission inhibited period is shaded in FIG. 2. During the transmission inhibited period, data transmission by a third-party terminal is inhibited and a DATA packet for transmitting data and an acknowledgement (ACK) packet for responding to the DATA packet are exchanged only between the initiator and the responder. Such a configuration can prevent collision between the DATA packet transmitted from the initiator and a packet transmitted from the third-party terminal.